law enforcement officials tell us it was flagged at a remote mail screening facility for mail sent to the white house. the f.b.i. says the letter mailed to the president was preliminarily tested positive for ricin a potentially lethal poison but it has to undergo further laboratory testing a day after another letter potentially laced with the chemical was sent to the office of a mississippi senator. that letter and both are dated april 8 and postmarked from memphis with similar wording. >> i remind you these investigations both of the bombings in boston and the letters in question here are just underway. i would point to the statement by the f.b.i. that they have made, they have no indication of a indication between the two. >> senate offices are on partial lockdown after the discovery of suspicious packages. the police investigation centers now on the offices of senator richard shelby in the russell senate building and joe manchin in the heart senate office building. the lockdown is unrelated to the incident involving the president's letter. >> thank you so much, we want to take you